I'm not worried so much about them being taught the sex ed side of AIDS (I knew what AIDS was before I knew it was sexually transmitted), rather the fact that teaching them about AIDS, or any other terminal disease, is likely to terrify them. They're, what... four? How is saying "Don't catch AIDS, you'll die!" going to do anything except scare them silly, and possibly make them fearful of AIDS patients in the future? Bad idea, and I really don't think there's any need.
They just not mentally developed and rational enough to comprehend it yet. I knew what AIDS was in elementary school (about seven or so), but I was much smarter than most kids, and as I said I had no idea it was an STD - my mother just told me you can get it from infected blood.
I think, if it's true, it's a poorly thought out idea. If you want to make sure your pre-school child is safe, just tell him/her not to touch other people's blood. You don't need to scare them.